The incommensurate-commensurate phase transition of CrMn single crystals is shown to be of first order. Coexistence of the two phases reported in the past is shown conclusively to be due to inhomogeneities in the Mn concentration of large samples. The observed relative length changes and latent heat effects at the transition correlate well with the known pressure dependence of the transition temperature. At this temperature very small resistivity anomalies are present as a result of a different temperature dependence of the resistivity of the incommensurate and commensurate phases.